1. Field of the Invention
The present invention relates to a probe head used on a coordinate positioning machine to orient a touch trigger probe (for example) relative to the movable arm (or quill) of the machine. The coordinate positioning machine may e.g. be either a coordinate measuring machine or a machine tool.
2. Description of Related Art
It is known (from "Probing for Productivity on Coordinate Measuring Machines" RENISHAW) to provide a two-axis motorised probe head for orienting a probe relative to the quill of a machine with two degrees of rotational freedom. Such a probe head comprises a support, which is connectable to the quill of a machine, and a pair of serially mounted rotors, each of which is engageable in, and may rotate between a plurality of indexed positions. A probe is connected to the "free" rotor, and is oriented relative to the quill by rotation of one or both of the rotors about their respective axes. Indexing of position between two relatively rotating parts is provided by a ring of balls on one of the parts, and three seating members on the other part, each of which seats in the convergent surfaces provided by an adjacent pair of balls. Thus, at each of the indexed positions each rotor is kinematically supported; this provides maximum positional repeatability of the probe at each indexed position.
It is often desirable, in order to perform different types of measuring operation, to exchange one probe which is carried by the probe head for another. EP 0142373 discloses a system which provides releasable kinematic engagement of a probe in order to facilitate probe change.
This obviates the need to calibrate the machine each time a given probe is removed and replaced on the machine, in order to determine the position of the probe relative to the quill.
However, in spite of the high positional repeatability of each of the individual kinematic locations formed between the probe and the quill for the purpose of supporting the probe, (i.e. the kinematic location of the probe on the probe head, and the kinematic location of each of the rotors in the probe head), the repeatability of the position of the probe relative to the quill in a given orientation of the probe head over a number of probe-change and/or probe head orienting cycles decreases with the number of kinematic locations formed between the probe and the quill which are coupled and uncoupled in the course of normal operation.